Performance of a large-scale multi-chip package system is highly dependent on communication capacity between integrated circuits, or communication capacity between packages on which the integrated circuits are installed. To attain a communication capacity of 10 to 100 Tb/s, a small-scale optical transceiver of low power consumption is required. As a modulated light source for modulating light is used in such a type of optical transceiver, the modulated light source is also required to be small in size and to be of low power consumption. As this type of modulated light source, a modulated light source formed of a semiconductor laser and a ring modulator is disclosed (see Patent Document 1, for example).
However, noise may occur in a modulated light source as disclosed in Patent Document 1, because the modulated light source may oscillate at multiple wavelengths, or an oscillating wavelength may be switched among multiple wavelengths. Such occurrence of noise in a modulated light source is not preferable because the noise may degrade communication performance.
The following is reference documents:    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2016-139741,    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2016-102926.